Sintering of nanostructured Sc2O3 ceramics from sol–gel-derived nanoparticles

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 3879–3887 www.elsevier.com/locate/ceramint

Sintering of nanostructured Sc2O3 ceramics from sol–gel-derived nanoparticles Nathalie Poirota,n, Damien Bregirouxb, Philippe Boyc, Cécile Autret-Lamberta, Philippe Bellevillec, Luc Bianchic a GREMAN, UMR 7347, Université F. Rabelais, Parc Grandmont, 37200 Tours cedex, France Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, Paris, France c Laboratoire Sol Gel, CEA/Le Ripault, BP 16, 37260 Monts, France

b

Received 5 October 2014; accepted 12 November 2014 Available online 21 November 2014

Abstract Sol–gel route was used to synthesize scandium oxides nanopowders. Oxohydroxide nanoparticles were first prepared in solution using ScCl3
xH2O as precursor. The influence of pH and reflux time on particle size and shape was studied. Sc2O3 nanoparticles were then obtained after water dialysis and ScOOH sol drying. Depending on pH, 40–1000 nm size ScOOH particles can be obtained. At a given pH, reflux time also influences the ScOOH particles size, which can vary from a few nanometers to 1 mm. The ScOOH sol can be used to prepare very pure Sc2O3 nanopowders. Results indicate a strong relation between nanoparticle size and the transformation temperature of γ-ScOOH to Sc2O3. A direct correlation between the Sc2O3 powder and the original ScOOH nanoparticle shapes has also been observed. From the TEM studies, it is likely that the crystallization from the phase ScOOH into Sc2O3 one is isomorphic. Spark plasma sintering test highlights a very good sinterability of the Sc2O3 powder. Nearly fully dense ceramics can be obtained at 1400 1C. Microstructure is very homogeneous with ultrafine grains, with size in the range of 25–225 nm. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Sc2O3 nanoparticles; Sol–gel synthesis; Spark plasma sintering; Nanostructure

1. Introduction The high melting-point cubic sesquioxide crystals (Ln2O3 with Ln¼ Y3 þ , Lu3 þ and Sc3 þ ) are attractive laser matrices due to their favorable properties, such as high transparency in a large frequency range, high thermal conductivity and the possibility of doping with rare earth ions. The short cation–cation distance and a very high density of cationic sites available for doping, allow the Sc2O3 system to be attractive to laser emission schemes, in particular to the energy transfer driven processes [1]. Laser operation of sesquioxides crystals doped with various RE3 þ (Ho, Tm, Er, and Yb) ions has been demonstrated [2]. Due to its high melting point (2420 1C), the Sc2O3 crystal growth from

n

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http://dx.doi.org/10.1016/j.ceramint.2014.11.067 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

melt is complex. Furthermore, the single crystal samples are limited in size, and are difficult to produce at an industrial level. Transparent ceramic materials are good alternatives to single crystals [3]. Scandium oxide ceramics have already attracted considerable interest due to their promising applications in optical components and solid state lasers such as high thermal conductivity, lower costs than single crystals, and good transparency. The preparation of doped and undoped transparent scandium oxide ceramics have already been reported [4–10]. Transparent Sc2O3 ceramics are usually obtained by vacuum sintering at high temperature (1700–1840 1C for 5–20 h). In these conditions, microstructures exhibit large grains in the range of 10–100 μm [11–13]. Bravo et al. observed the influence of the powder processing on the sintering of Ybdoped Sc2O3 ceramics. Their results show that nanoparticles obtained by wet chemical routes are more effective for the fabrication of transparent ceramics. Nevertheless, it is possible

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to achieve good transparency from solid state reaction derived powders, but high energy ball milling and addition of sintering aids are required [12]. Jiang et al. obtained fully dense Sc2O3 ceramics with huge grains (4100 μm) at 1840 1C with CaO as sintering aid [14]. Futami et al. obtained translucent Sc2O3 ceramics with grain size between 1 and 3 μm by using the spark plasma sintering technique (SPS) [15]. Sintering parameters are not specified. But it is well known that SPS allows achieving full densification at lower temperature and shorter time than by conventional sintering. This technique was already successfully used for the fabrication of rare-earth sesquioxide ceramics with high transparency [16–18]. One of the keys to obtaining highly dense Sc2O3 ceramics (4 99%) is the control of inter-grain pore density. Furthermore, it is well known that decreasing powder size can lead to lower sintering temperature. Indeed, in a densification process governed by solid state diffusion, the densification kinetic is inversely proportional to the particle radius. These parameters determine the degree of transparency. In consequence, particle shape, size and agglomeration state control is essential. To prepare nanocrystalline Sc2O3 powders, different routes can be used. The oxalate precipitation technique with pure ethanol enables the preparation of well-dispersed Sc2O3 particles with high specific surface area [19]. Another technique recently studied, called propellant synthesis, is based on solution combustion [20,21]. Here, the as-formed scandia samples are constituted of aggregated single crystal particles with non-regular shape, and size distribution usually ranging from 20 to 40 nm. The small voids inside some particles are generated by the large amount of gas produced during the rapid combustion reaction. As a consequence, high combustion reaction temperatures strongly influence the particle shape and consequently the sintering conditions. Sc2O3 powders can also be prepared at a lower temperature using other techniques such as electrochemical deposition, reverse-strike precipitation and hydrothermal synthesis [22,24]. Christensen et al. and Milligan et al. reported hydrothermal preparation of ScOOH with boehmite structure (γScOOH) and diaspore structure (α-ScOOH) [25,26]. To elaborate high performance Sc2O3 laser ceramics, the synthesis of high quality starting powders is a key point. The morphological characteristics and the purity degree of the starting powders play key factors on the properties of the final materials. The main objective of this work is to prepare oxohydroxide nanoparticles at 100 1C with controlled size and shape by a sol–gel route, and to dry them at a higher temperature to obtain scandium oxide Sc2O3. The objective size is lower than 20 nm with a weak dispersion. The sol–gel process enables the preparation of oxide nanoparticles directly in solution, without additives. Good oxide homogeneity, high purity and shape and size control are some advantages of the solution process. Grosso et al. obtained colloidal particles of ScOOH by the sol– gel method [27]. ScOOH-based platelet-lozenge shape particles of 66 nm length and 37 nm width have been produced from Sc(acac)3 dissolved in alcoholic solution. Li et al. have previously studied the influence of Sc precursor preparation on

Sc2O3 powder using the wet-chemical route [8,9,28,29]. Monodispersed Sc2O3 precursor particles are prepared via homogeneous precipitation from various Sc precursors (nitrate, chloride, and sulfate). NO3  and Cl  lead to ScOOH after precipitation with urea at 90 1C. ScOOH is obtained by the precipitation of scandium nitrate in ammonia whatever be the preparation conditions (reaction temperature up to 70 1C, aging and pH). In this work, a new route of ScOOH preparation is proposed, with ScCl3 as precursors. ScOOH is produced by precipitation with NaOH and reflux in water at 100 1C at constant volume. In these conditions, no hydrothermal process is necessary, only pH control and reflux-time duration are required. Furthermore, the salt elimination is processed by water dialysis, which, depending on pH and reflux-time, leads to a purer stable ScOOH sol. The influence of pH and preparation time on ScOOH size and shape is clearly shown, as well as the impact of ScOOH preparation conditions on final Sc2O3 features. Finally, the sintering behavior, i.e. densification and final microstructure, of the synthesized powder was investigated by using the spark plasma sintering (SPS) technique. Results are then compared to previous data found in literature.

2. Experimental The oxohydroxide nanoparticles were produced with scandium chloride hexahydrate (4 99.9% pure, Alfa Aesar) dissolved in water with a molar ratio R=[ScCl3]/[H2O] around 160. The initial pH of the chlorhydrate aqueous solution was around 3. The condensation/peptization reaction leading to ScOOH sol was processed by adjusting the pH with 10% NaOH solution at room temperature, and then refluxing the ScOOH solution at 100 1C and pH 4 6. Various pH and reflux time have been studied respectively from pH 7 to 11 and 2 to 24 h. pH monitoring was performed during the addition of base as the Point of Zero Charge (PZC) of ScOOH and Sc2O3 is respectively close to 5.7 and 6.7 [30]. In each case and before the step reflux, a white precipitate was formed. After reflux, the resultant suspension was dialyzed in water to remove salts. The dialysis water was changed 4 times. The ScOOH particle size and their distribution were measured by laser granulometry (DLS) (Malvern Nano ZS) after dispersion in an ultrasonic bath for 15 min. Excess water in the ScOOH sol was further removed by drying at 100 1C in air. The sol was then thermally treated at above 400 1C in a furnace to yield Sc2O3 powder. Thermogravimetry and differential thermal analyses (Diamond TG/ DTA analyzer) of dried precursor were performed at a heating rate of 10 1C min  1, from room temperature to 1000 1C, using α-Al2O3 as reference. The phase identification is performed by X-ray diffractometry with a Rikagu diffractometer in a θ–2θ geometry using Cu-Kα radiation (λ ¼ 1.5406 Å). Powder purity was checked by FTIR analysis (Nicolet, 550 series II spectrometer). The particle morphology was observed by transmission electron microscopy (JEOL 2100, FEG-TEM 200 kV) on a copper grid after dispersion in an alcohol solution.

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Fig. 2. XRD patterns of γ-ScOOH of ambient-dried sols obtained at pH 7 and 9 after a 4 h reflux time. Gray circles indicate the presence of diffraction peaks of XRD sample support.

Fig. 1. Thermal analysis of ScOOH annealed at 100 1C. Derivatives of DTA and TG signals are represented in the inset.

(631) (444)

(541)

(611)

(433)

(431)

(512)

(322)

(411)

(321)

(400)

(211)

Intensity (a.u.)

500°C

10

20

30

40

50

(231)

(020)

(051)

(120)

100°C

(031)

300°C

(151)

400°C

(020)

Thermal analysis on ScOOH sols synthesized at pH 7, 9 and 11 with a 4 h reflux time, and dried at 100 1C in air for 1 h, have been made. Only the results for pH 7 are shown in Fig. 1, the same thermal behavior being observed for pH 9 and 11. ScOOH thermal decomposition proceeds through three steps. The first step, between 100 and 200 1C, corresponds to solvent water evaporation. The second one, between 190 and 320 1C, is associated with the release of the γ-ScOOH excess water, and corresponds to a 6.44% weight loss. This step indicated that the γ-ScOOH precursor is hydrated and can be expressed by γ-ScOOH
0.28 H2O. The last step is attributed to the conversion of γ-ScOOH into Sc2O3 with a one-step weight loss of about 9% below 530 1C [24]. This step corresponds to the γScOOH hydroxyl group loss to produce Sc2O3. On the DTA curve, an endothermic reaction peak at 457 1C is observed corresponding to Sc2O3 formation (see inst in Fig. 1).

(622)

3.1. Synthesis of precursors and Sc2O3 nanoparticles

(543) (640)

3. Results and discussion

(440)

(222)

Sintering test was performed at 1400 1C for 1 min under 0.1 mbar vacuum by spark plasma sintering technique (Dr. Sinter 515S Syntex machine). The Sc2O3 powder was filled into a graphite die with an inner diameter of 8 mm. Temperature was monitored by an optical pyrometer from 600 1C to the sintering temperature. Sample shrinkage was followed by the displacement of the lower punch. Heating rate, cooling rate and applied pressure were set to 100 1C min  1, 50 1C min  1 and 100 MPa, respectively. Pressure was applied gradually from room temperature to 600 1C and maintained at its maximum value until the beginning of the cooling step. Pressure was then removed in 5 min. Sintered sample was then annealed under flowing air at 1100 1C for 10 h in order to remove carbon contamination from the graphite die and oxygen vacancies generated by the reducing environment during sintering. Microstructure was observed by FEG-SEM on a thermally etched surface. Grain size distribution was determined by imaging analysis of 567 grains.

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60

70

2θ (degree)

Fig. 3. Evolution of ambient XRD-patterns of γ-ScOOH with calcination temperature. The reference XRD-pattern for cubic Sc2O3 is included for comparison (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. FTIR spectra of γ-ScOOH after different thermal treatments. The FTIR peaks below 500 cm  1 are assigned to vibrational modes of Sc–O bond.

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In Fig. 2, only the XRD patterns of γ-ScOOH for pH 7 and 9 after drying the sol at ambient temperature have been shown. The sol was obtained at pH 7, 9 and 11, after a 4 h reflux time. The system is well crystallized and indexed to γ-ScOOH with an orthorhombic structure regardless of pH (JCPDS no17-941, Amam) whatever be the pH value. This structure is found to be isomorphous with lamellar lepidocrocite γ-FeOOH and is

Fig. 5. TEM micrographs of particles for pH 7, thermal treatment of 500 1C, and reflux time of 4 h (a), 24 h (b) and 72 h (c).

consistent with anisotropic lozenge plate-like shape [26]. No α-ScOOH phase was detected. γ-ScOOH was previously reported to form under ambient conditions using precipitation of nitrate solution from ammonia water, by a hydrolysis of Sc (acac)3 or by a hydrothermal method [24–28,30]. Our results indicate that a γ-ScOOH stable sol is directly obtained by hydrolysis/condensation of ScCl3 in water using NaOH as a base. In order to obtain Sc2O3 nanopowder, the stable γScOOH sol was annealed at different temperatures. The X-ray diffraction patterns (XRD), after 1 h heat treatment at 100, 200, 300, 400 and 500 1C on the ScOOH nanopowder obtained at pH 7 with 4 h reflux time, are reported in Fig. 3. XRD patterns of dried ScOOH at 100 and 200 1C being similar, only the diffractogram at 100 1C has been shown in Fig. 3. Below 400 1C, the main phase is orthorhombic γ-ScOOH. Between 300 and 400 1C, a new peak appears as indicated by the filled circle. The latter can be attributed to the (222) plane of the Sc2O3 cubic phase, whereas the peaks attributed to the γScOOH phase disappear gradually. At 500 1C, only the cubic Sc2O3 phase is observed with an Ia3 space group. By

Fig. 6. TEM micrographs of the ScOOH sample, prepared at pH 7 with a reflux time of 4 h and annealed at 100 1C, before (a) and after annealing at 500 1C (b). Insets are the selected-area electron patterns (SAED).

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comparison with the Sc2O3 nanopowder prepared by a homogeneous precipitation method, Sc2O3 appears at a lower temperature. Germination step begins between 300 and 400 1C. The same studies carried out on ScOOH synthesized at pH 9 and 11 show similar results. Fig. 4 shows FTIR spectra against temperature when ScOOH is prepared at pH 7 with a 4 h reflux time. γ-ScOOH, α-ScOOH, and Sc(OH)3 can be discriminated on IR spectra due to the OH and Sc–O–Sc vibration bands [31]. It could be noted that no scandium carbonate is formed even when solutions are drying in air. In our case, samples exhibit only typical absorption bands of γ-ScOOH precursor. No α-ScOOH and Sc(OH)3 absorption bands are observed, in accordance with XRD data, whatever be the temperature. The absorption bands at 3216 cm  1 and 3073 cm  1 are typical of hydroxyl stretching (νOH) whereas the δOH hydroxyl bending modes appear at 1084 cm  1 and 937 cm  1. The absorption band at 1640 cm  1 is characteristic of H–O–H bending mode in water. The γOH bands are located below 500 cm  1. At 400 1C, a vibration mode appears at 633 cm  1 attributable to Sc2O3. At this temperature, two compounds remain present: γ-ScOOH and Sc2O3. At 500 1C, only

Fig. 7. TEM micrographs of particles obtained at pH 9 for reflux time of 4 h (a) and 24 h (b).

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the absorption bands of the Sc2O3 are observed. These results confirm the previous data obtained by XRD and DTA/DSC which indicate that the Sc2O3 formation temperature is below 500 1C. The same study has been carried out at pH 9 with 4 h reflux time. Results are similar to those obtained at pH 7. XRD studies indicate that the gradual transformation from γ-ScOOH into Sc2O3 takes place between 300 and 500 1C. Whatever be the pH value and annealing temperature, no α-ScOOH signature is observed. As pointed out before, whatever be the pH and reflux time, only characteristic γ-ScOOH absorption bands are observed. This work is in agreement with the observation of Ripert et al. who observed only the γ-ScOOH phase, prepared by hydrothermal syntheses (200 1C, 10 h) using ScCl3
6H2O as starting material [30]. Here the conversion of the ScOOH phase into Sc2O3 does not follow the order γ-ScOOH into α-ScOOH as observed by Zhang et al. [24]. One can suggest that the formation of the γ-ScOOH phase is neither influenced by the pH nor by the reflux time, but rather by the nature of the initial precursor and of the complexant. The crystalline shape and size of the powder grains heat-treated at 100 1C and 500 1C for various reflux times (4–72 h) and pH (7, 9 and 11), were determined by TEM. This study reveals that the Sc2O3 particles are generally an agglomeration of smaller grains with an average size below 20 nm, except at pH¼ 11. This result is consistent with the X-ray line-broadening analysis from the Debye–Scherrer equation on the Sc2O3 powders obtained at

Fig. 8. TEM micrographs of ScOOH obtained at pH 9 with a reflux time of 4 h and annealed at 100 1C (a) and 500 1C (b). The insets are the selected area electron diffraction patterns (SAED).

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pH 7 and 9, which gives an average crystallite size of 10–15 nm. It can be stressed that nanocrystallites observed by TEM are only lightly agglomerated. The Sc2O3 grains can then be easily dispersed in a solvent. TEM micrographs in Fig. 5 show the influence of reflux duration at pHi ¼ 7 on the Sc2O3 particle morphology after heating at 500 1C. For a reflux duration of 4 h (see Fig. 5(a)), the presence of disk or hexagonal-like grains with size o20 nm can be noted. For reflux time of 24 h and 72 h (Fig. 5(b) and (c)), nanorods, disk-like and hexagonal-like nanoparticles with a 10– 30 nm size, are observed. Nanorods are favoured when the reflux time is increased. This geometry is not the best one to obtain dense ceramic from powder. A reflux time of 4 h, leading to disklike and lozenge-like nanoparticles, should then be preferred. To study the influence of the ScOOH shape on the morphology of Sc2O3, TEM micrographs have been compared of samples treated at 100 1C (ScOOH) and 500 1C (Sc2O3) with the same reflux time (4 h) and pH 7. The ScOOH solution dried at 100 1C shows disk-like plates with sizes between 5 and 10 nm (Fig. 6(a)). The same sample, with a thermal treatment at 500 1C, exhibits disk and hexagonal-like grains with a size around 10 nm (see Fig. 6(b)). Selected area electron diffraction (SAED) evidences well-crystallized nanocrystals when the samples are heated at 500 1C. At 100 1C, the SAED shows amorphous and crystallized areas. Observation of an

amorphous zone can be correlated to the presence of hydrated phase in the sample, as observed by TGA/DTA. The TEM micrographs in Fig. 7 show the Sc2O3 particle morphology and size at pH 9 for reflux times of 4 h and 24 h with a thermal treatment at 100 1C. As observed for the pH 7, the particle size is less than 20 nm. Hexagonal-like and disklike shapes are observed in each case with needle-shape agglomerated particles. As for pH 7, the influence of the shape of ScOOH on the morphology of Sc2O3 has been studied. TEM micrographs were performed on samples treated at 100 1C and 500 1C, with a reflux time of 4 h. The product dried at 100 1C (ScOOH) exhibits needle-shape agglomerated particles of around 20  2 nm2 (see Fig. 8) and disk and hexagonal-like grains. When samples are heated at 500 1C (Sc2O3), a majority of the nanorods, with sizes around 20  2 nm2 and hexagonal-like particles with sizes below 10 nm, are observed. This result is in accordance with the results reported by Grosso et al. [27]. By using a sol–gel method, they reported the same lozenge shape platelets of scandium oxo-hydroxo particles with the Sc(acac)3 as starting material. To confirm the influence of pH and reflux time on the morphology of nanoparticles, a TEM study at pH 11 was started to obtain more comprehensive results. TEM micrographs of Sc2O3 particles, prepared at pH 11 with a reflux time of 4 h and 24 h, are

Fig. 9. TEM micrographs of Sc2O3 after annealing at 500 1C of ScOOH, obtained by reflux at pH 11 with reflux time of 4 h (a, b), and 24 h (c, d).

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densification by conventional vacuum sintering [11–13]. The final relative density is found to be 98% of the theoretical one. Typical resulting microstructure is shown in Fig. 12. Mean grain size is found to be around 100 nm, meaning that grain growth remains quite negligible during sintering. Few isolated pores of around 40 nm were found at some triple points in the microstructure. It appears clearly that densification occurs in two steps, since the displacement rate curve exhibits two distinct peaks, one at 1050 1C and the other one at 1235 1C. The first peak can be attributed to the densification of primary nanoparticles, whereas the second one reflects the densification of agglomerates. Indeed, it is well known that agglomerates retard densification. By extrapolating the first shrinkage step, a full densification could be expected to occur at around 1250 1C if sintering is performed from a perfectly dispersed powder. This could be achieved by using pre-shaping process by slip-casting in order to make homogeneous and big pore free green compacts before SPS sintering [17–32]. 4. Conclusion A new sol gel route for obtaining nanoparticles of Sc2O3 was proposed with ScCl3 as scandium precursor. From the above results, it is clear that the ScOOH can be obtained at low temperature and pressure conditions. Hydrothermal treatment is not necessary to obtain small and well-dispersed 460

Crystallisation temperature (°C)

shown in Fig. 9. Grain size is around 30 nm. Compared to those at pH 7 and 9, the grains are more agglomerated in accordance with laser granulometry measurements. Whatever be the reflux time, the TEM micrographs indicate the presence of very small nanorods, needles. Hexagonal-like and disk-like nanoparticles are present. The comparison between particles size measured by TEM and by DLS for various synthesis conditions evidences a strong agglomeration at pH¼ 11, whereas grain size is quite similar. In Table 1, particle size measured by DLS and TEM according to pH value has been compared. In conclusion, the γ-ScOOH precursor and Sc2O3 consist of regular-shaped particles with narrow size distribution. The shape of ScOOH or Sc2O3 seems to be dependent on the pH value giving nanorods, disks-like nanoparticles or both. The proportion of nanorods varies with pH and reflux time. For pH 11, the particles are more agglomerated, which can be explained by the presence of more nanorods-like grains compared to pH 7 and 9. It is worth to note that the ScOOH to Sc2O3 conversion temperature increases when crystalline size of ScOOH increases for pH 7 and 9 (Fig. 10). Present results are in accordance with data found in the literature, where the presence of hexagonal-shaped platelets is attributed to γ-ScOOH, prepared from ScCl3
6H2O [30]. On the contrary, for Sc(NO3)3 as starting material, Zhang et al. claimed that this particle shape is attributed to the presence of α-ScOOH phase [24]. In both cases, hydrothermal conditions are used. In our paper, ScCl3
6H2O is the starting material. We can conclude that the nature of the scandium salts determines the formation of the γ-ScOOH or α-ScOOH phase and not the synthesis method. Furthermore, nanocrystallite shape is not changed when γ-ScOOH is transformed into Sc2O3, like in a pseudomorphic change. The oxo-hydroxide morphology (shape, size) seems to remain the same for Sc2O3 after drying at 500 1C. This observation has been already done by Xiu et al. [19]. Whatever be the value of pH and the reflux time, except at pH 11, the particles are quite uniform in size and are only loosely agglomerated. This should be beneficial for sintering.

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450

440

430

420

0

50

3.2. Sintering test

Table 1 Influence of pH on the particle size measured by DLS and TEM.

5 10 5 5 6

150

200

pHinitial

7 7 9 11 13

pHfinal

5.5 5.5 7 9 11

Particle size (nm) TEM

DLS

20 20 20 30 –

60 80 70 4 1000 4 1000

Fig. 10. Correlation between the particle size of ScOOH precursor obtained by DLS and crystallization peak obtained by DTA.

2

Punch displacement (mm)

Fig. 11 shows the lower punch displacement during the heating step of the SPS sintering of the Sc2O3 powder (pH 7, reflux time of 4 h) that reflects the densification of the sample. This indicates that sample is nearly fully dense at a temperature as low as 1350 1C, which is at least 350 1C lower than that required for full

Concentration of ScOOH sol (%)

100

Particle size (nm)

1.5

1

0.5

0 600

700

800

900

1000 1100 1200 1300 1400

Temperature (°C)

Fig. 11. Punch displacement as a function of temperature during SPS sintering of Sc2O3 nanopowder. The dotted line shows the punch displacement rate.

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Fig. 12. Typical microstructure of Sc2O3 ceramic sintered by SPS at 1400 1C—1 min and associated grain size distribution (in number).

particles. As confirmed by XRD, FTIR and DTA/DSC studies, after drying at 100 1C, the resulting colloidal sol product is the γScOOH phase, while annealing at 500 1C enables the formation of the pure Sc2O3 compound. This work shows that the control of ScOOH and Sc2O3 nanoparticle size and shape can be achieved by controlling pH and reflux time duration. Results confirm the previous published data that show that pH is then the main factor controlling the size and shape of nanoparticles. The best synthesis conditions are found to be pH 7 and 4 h reflux time. In these conditions, ScOOH agglomerates are monodispersed and have an average size below 70 nm. TEM studies indicate that the crystallite shape is hexagonal and disk-like, with a grain size between 5 and 10 nm. At pH 11, the system is more agglomerated (41000 nm) with a grain size above 30 nm. The γ-ScOOH into Sc2O3 phase formation is isomorphic and its temperature is dependent on particle size, but independent of pH. This study illustrates the complexity of the problem related to the preparation, in a predictable way, of uniform particles of a given chemical composition and shape by the precipitation process. It was possible to produce uniform particles of different shapes by changing only two parameters in our case, the pH and the flux time. All these results are significant because the precursor shape plays a key role in the control of the sintering conditions. Spark plasma sintering tests performed on powder obtained from sol at pH 7 with a reflux time of 4 h highlight a very good sintering behavior. After sintering at 1400 1C for 1 min, Sc2O3 ceramics are nearly fully dense and have a mean grain size of around 100 nm. These two values are both pretty much lower than those found in the literature. Improvement of the prior shaping step should allow the fabrication of ultrafine grains transparent Sc2O3 ceramics. References [1] A. Lupei, V. Lupei, C. Gheorghe, A. Ikesue, Excited states dynamics of Er3 þ in Sc2O3 ceramic, J. Lumin. 128 (2008) 918–920. [2] C.R.E. Baer, C. Kränkel, O.H. Heckl, M. Golling, T. Südmeyer, R. Peters, K. Petermann, G. Huber, U. Keller, 227-fs pulses from a mode-locked Yb:LuScO3 thin disk laser, Opt. Express 17 (13) (2009) 10725–10730. [3] S.F. Wang, J. Zhang, D.W. Luo, F. Gu, D.Y. Tang, Z.L. Dong, G.E. B. Tan, W.X. Que, T.S. Zhang, S. Li, L.B. Kong, Transparent ceramics: processing, materials and applications, Prog. Solid State Chem. 41 (2013) 20–54.

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